Polycarbonate polyols have enjoyed increasing popularity in the last few years. For instance, these compounds can be used at least to some degree in place of polyether polyols, especially in the production of polyurethane foams, especially flexible foams. Carbon dioxide is used as a comonomer in polycarbonate polyols. It is not just less expensive than the alkylene oxides typically used for polyether polyols, such as ethylene oxide or propylene oxide, which are usually of petrochemical origin; in addition, through the incorporation of carbon dioxide into the polymer, a greenhouse gas is simultaneously used, which is of interest from an environmental point of view.
Processes for preparing polycarbonate polyols are known in principle. These are generally produced from alkylene oxide and carbon dioxide proceeding from a starter substance with use of suitable catalysts. A distinction is made essentially between two preparation processes. In the first variant, alkylene oxide units and the carbon dioxide used as comonomer are built up in a very substantially alternating manner to form a polymer chain. The starting use for this reaction may be a short-chain polyol, especially a diol, for example ethylene glycol. Such a preparation process is known, for example, from WO 2010/028362 A1.
Another preparation process is known, for example, from WO 2008/092767 A1. In this known preparation process, a polymer is likewise formed proceeding from a short-chain starter, such as ethylene glycol or glycerol, with use of what are called double metal cyanide catalysts (DMC catalysts), and additionally has ether moieties in the polymer chain as well as the carbonate groups formed from alkylene oxide and carbon dioxide. This is achieved by virtue of the ability of DMC catalysts to catalyze not just the reaction of alkylene oxide with carbon dioxide but equally ether formation through reaction of two alkylene oxides with one another. In this way, it is possible to adjust the profile of properties of the polymers obtained with considerably greater flexibility.
In the aforementioned preparation processes, however, as well as the actual polymer chain formation, there are typically unwanted side reactions that lead to products of low molecular weight. For instance, reaction of one alkylene oxide molecule with one carbon dioxide molecule can result in formation of cyclic alkylene carbonates, and in the formation of propylene carbonate, for example, when propylene oxide is used. The formation is shown in the following formula (I) (for example with R═CH3 propylene carbonate):

Since the presence of these compounds in the end product is generally undesirable, the reaction in the polymerization is conducted under conditions under which a minimum amount of cyclic alkylene carbonate forms. This can be achieved in a manner known per se by observing particular reaction temperatures. This is described, for example, in WO 2008/092767, where 80-130° C. is specified as the preferred reaction temperature. Nevertheless, it is not possible even by these measures to entirely suppress formation of cyclic alkylene carbonates. For example, polyether carbonate polyols prepared by means of DMC catalysis have a proportion of 3% to 15% by weight of cyclic alkylene carbonate. The proportion of cyclic alkylene carbonates present in the polycarbonate polyols has to date been reduced to a content of below 1% by separation processes after the preparation.
WO 2008/092767 describes the removal of cyclic propylene carbonate (cPC) by means of vacuum stripping at 150° C. (3 hours), giving residual cPC contents of <200 ppm in the end product. There is additional description of the removal of cPC by means of thin-film evaporators or falling-film evaporators under high vacuum (pressure <1 mbar) at 120° C. (see BMS 11 1 160-EP; application no. 12181907.7). Residual cPC contents are unspecified here.